1. Field of the Invention
The present invention relates generally to hydrostatic transmissions and more specifically to the control structure for shifting the swash plate of a hydrostatic transmission pump between its forward, neutral and drive orientations. More particularly the invention relates to a structure for precisely locating the swash plate in its neutral position whereby "creep" or small movements of the vehicle can be controlled.
2. Description of the Background Art:
Directional control of a vehicle having a hydrostatic transmission can be accomplished by shifting the swash plate of the transmission's hydraulic pump between first, second and third positions to operate the transmission motor in either its forward, neutral or reverse direction.
Since the swash plate controls the hydraulic fluid flow over a continuous range, very small incremental changes in flow rate are possible with small movements of the swash plate. When an operator wants to maintain the machine in an idle or neutral configuration, it is important that the swash plate be precisely located at its neutral or non-flow condition to avoid "creep" or slight movement of the vehicle.
In many transmissions, the orientation of the swash plate is controlled through rotating a shaft which is coupled with it. Commonly the shaft is fitted with a hub that is coupled with a control lever which can be rotated by directional control structures such as foot pedals to shift the swash plate between its forward, neutral, and reverse orientations.
Since, the swash plate must be returned to a "neutral" orientation to prevent vehicle movement, neutral locating structures, such as V-type cam surfaces utilizing cam rollers are also coupled with the control lever. These locating structures are provided to return the swash plate to the precise "neutral" position after the directional foot control pedals are released.
It is therefore important to provide a reliable linkage between the swash plate and the linkages connected with the control lever to assure that the plate is returned to its exact "neutral" orientation when desired. Since the orientation of the hub and lever are determined by the neutral locating structure, it is important that the hub be precisely coupled to the swash plate shaft as well as to the lever. To assure this relationship, keyway couplings have been provided between the shaft and hub. Further, the shaft and hub openings have been tapered to insure a tight fit between them. It is also important that the hub be accurately and precisely joined to the lever which is shifted by the neutral locating structure.
To manufacture the hub and lever structures so that they are precisely oriented with respect to one another, hubs have first been welded to the levers and then had the tapered and keyway openings machined into the hub. This is a very expensive, time-consuming procedure that can also require separate hub-lever combinations for different vehicle models.
To overcome this expense, it would be desirable to provide a hub that would precisely fit both the swash plate shaft and various lever sizes or configurations without requiring that the hub first be welded to a lever and then have its openings machined.